Case IH t77 – PCU: Control Duration, Shorted To High Source

An electrical fault where a control circuit is experiencing a short circuit to a high voltage source (typically battery voltage), causing the control signal's duration to remain high longer than the system commands.

• MXM
• MXM 120
• MXM 130
• MXM 140
• MXM 155
• MXM 175
• MXM 190

1. 1

   Check the wiring harness connecting to the PCU for any visible chafing, melting, or damage that could cause a short to battery voltage.

2. 2

   Use a multimeter to test the specific control circuits for a short to a high source. Disconnect the controller and check the pins against ground and battery voltage to isolate the short.

3. 3

   Inspect the system fuses (such as fuse 13). If a fuse is blown, replace it with a correctly rated fuse and test the system. If it fails again, a short to ground or high source is confirmed.

4. 4

   Enter the tractor's H-Menu to perform a clutch pressure test (Mode HJ). This allows you to energize transmission clutches (A, B, C, D, and E) independently to see which circuit is failing or hanging open.

1. 1

   Isolate and Repair

   Disconnect the battery. Locate the shorted wire in the PCU harness and repair or replace the affected wiring section.

2. 2

   Component Replacement

   If the wiring is intact, the fault likely lies within a failed PWM solenoid valve or the PCU microprocessor itself. Replace the faulty component.

3. 3

   Transmission/EDC Calibration (Mandatory)

   A Transmission/EDC Calibration is required if the microprocessor (PCU) has been replaced, a PWM solenoid valve has been replaced, a potentiometer has been replaced, or if the non-volatile memory has been reset.

4. 4

   Calibration Steps

   Start the engine and set the engine speed to 1200 ERPM (± 100). Press and hold the High and Low range buttons until 'CAL' appears on the transmission display, then release. Proceed through the clutch calibration sequence until 'END' is displayed, signifying completion.